Basically, the wireless LAN supports a basic service set (BBS) including an access point (AP) and a plurality of wireless stations excluding the AP. The AP serves as an access point of a distribution system (DS). Hereafter, the AP and the stations are commonly called ‘station’.
According to the IEEE 802.11n standard, when a station receives a data frame, the station transmits an acknowledgement (ACK) signal after a short inter frame space (SIFS) in order to increase the transmission efficiency in a media access control (MAC) layer, even though there is a difference depending on a policy for an ACK signal required by the corresponding data frame.
FIG. 1 is a timing diagram explaining data transmission in the MAC layer according to the IEEE 802.11 standard.
When a station 1 transmits a data frame 101 to a station 2, the station 2 receives the corresponding data frame and then transmits an ACK signal 111 after an SIFS which is a predetermined time. Such a method shown in FIG. 1 is frequently used in the MAC layer of the wireless LAN.
Meanwhile, as the number of users using the wireless LAN has rapidly increased, the request to improve data throughput provided by one BSS is increasing. In the existing wireless LAN, one station may communicate with only one station at a certain moment. However, research has been conducted on technology which enables one station to communicate with several stations at the same time, in order to provide a gigabit or more throughput. As the representative technology, a multi-user MIMO (hereafter, MU-MIMO) scheme and a multi-frequency channel scheme are provided.
When those schemes are used, one station may operate as if the station transmits and receives frames to and from several terminals through several independent communication paths, respectively. Accordingly, the station may transmit data to several stations at the same time. As a result, it is possible to significantly increase the throughput of the BSS.
However, when using the several communication paths at the same time, the station has a limitation in which the transmission and the reception cannot be performed at the same time through all the used communication paths. For example, when a certain station uses a communication path 1 and a communication path 2 at the same time, the communication path 1 cannot be used for transmission in case where the communication path 2 is used for reception.
All data frames used in the wireless LAN have a variable length. As described above, an ACK or a block ACK is transmitted immediately after a predetermined time passes from a time point when the reception of data frames is completed. Therefore, when data frames are simultaneously to several stations through several communication paths, the respective stations will transmit an ACK immediately after a predetermined time passes from a time point when the reception of the data frames having different lengths was completed. That is, a station having received a data frame having the shortest length may transmit an ACK, before the transmission of the data frames to the other stations is completed. In this case, the corresponding ACK may not be received.
This will be described with reference to FIG. 2.
In FIG. 2, it is assumed that stations 1 and 2 exist and data are transmitted between the stations 1 and 2 through different communication paths. That is, when the station 1 transmits a data frame 201 and a data frame 202 having different lengths, the transmission of the data frame 201 having a shorter length may be first completed. In this case, when the length of the data frame 201 is smaller than that of the data frame 202 by an SIFS 221 or more, the station 2 transmits an ACK 211 immediately after the SIFS 221 passes from a time point when the reception of the data frame 201 is completed.
However, since the station 1 is still transmitting the data frame 202 at the time point when the station 2 transmits the ACK 211, a reception impossible section 230 occurs, in which the station 1 cannot receive the ACK 212 transmitted by the station 2.